SCIENCE PROJECT

CHEMICAL REACTIONS

A chemical reaction is a process that always results in the inter-conversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants. Chemical reactions are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants. Classically, chemical reactions encompass changes that strictly involve the motion of electrons in the forming and breaking of chemical bonds, although the general concept of a chemical reaction, in particular the notion of a chemical equation, is applicable to transformations of elementary particles, as well as nuclear reactions.

SIGNS OF A CHEMICAL REACTIONBubbles of gas appear. Gas-producing reactions run to completion when the gas can leave the reaction mixture. Gaseous products appear as bubbles only after the reaction mixture has become saturated with the gas. If there are no rough surfaces or dust particles to nucleate the bubbles, the reaction may produce a supersaturated gas solution. Bubbles can also be produced when the liquid boils, or when air dissolved in the liquid comes out of solution as the liquid is warmed.A precipitate forms. When mixing a pair of soluble reactants in solution, the sudden appearance of a solid that 'rains down' (precipitates) into the bottom of the container is a sign that a reaction has occurred. Sometimes the precipitate particles are too small to settle out; in this case, look for a cloudy solution. If the amount of precipitate is very small, you can sometimes detect fine particles of precipitate suspended in the solution by shining a

very bright light through the liquid. If you can see the beam of light in the solution, fine suspended particles are present. For reactions that occur in a melt, don't confuse precipitation with simple freezing.A color change occurs. Every compound absorbs a characteristic set of colors of light. This absorption spectrum is a chemical fingerprint for detecting the presence of that compound. When the compound is altered in a chemical reaction, the fingerprint will change- and so the color of the reacting mixture may change, as the reaction progresses.The temperature changes. It takes energy to break chemical bonds. And energy is released when new chemical bonds form. When the reaction involves more bond-breaking than bond making, the energy required is often absorbed from the surroundings, making them cooler. When there is more bond-making than bond-breaking, the excess energy is released, making the surroundings hotter.Light is emitted. Sometimes energy is released by bond-forming reactions in the form of light. This occurs in most combustion reactions. Living things that glow in the dark- such as fireflies, funguses, and deep sea creatures- produce light without heat, using chemical reactions.A change in volume occurs. Density is a characteristic of a compound, and if new compounds are produced as the compounds are consumed in the reaction, the change in density can cause the reacting mixture to expand or contract as the reaction proceeds. Sometimes this volume change can be large and very rapid- and an explosion occurs!A change in electrical conductivity occurs. Some reactions produce or consume ions (charged particles) in a solution. Changes in the character and concentrations of the ions will cause the reacting mixture's ability to conduct electricity to change.A change in melting point or boiling point occurs. The melting or boiling point is characteristic of a compound; when the composition of a mixture changes, the melting point and boiling point also change.A change in smell or taste occurs. Since many chemical reactions have poisonous reactants or products, this method of detecting chemical change isn't recommended!A change in any distinctive chemical or physical property occurs. Pick a property that uniquely characterizes one of the compounds involved in the suspected reaction, and monitor it. If the property really distinguishes that compound from all the others, you'll see it change when a reaction occurs.

CHEMICAL EQUATION

A chemical equation is a way to describe what goes on in a chemical reaction, the actual change in a material. Chemical equations are written with the symbols of materials to include elements, ionic or covalent compounds, aqueous solutions, ions, or particles. There is an arrow pointing to the right that indicates the action of the reaction. The materials to the left of the arrow are the reactants, or materials that are going to react. The materials to the right of the arrow are the products, or materials that have been produced by the reaction. The Law of Conservation of Mass states that in a chemical reaction no mass is lost or gained. The Law of Conservation of Mass applies to individual types of atom. One could say that for any element, there is no loss or gain of that element in a chemical reaction. There are such things as reversible reactions, reactions in which the products reassemble to become the original products. Reversible reactions are symbolized in chemical equations by a double-headed arrow, but the standard remains to call the materials on the left the reactants and the materials on the right the products.

EXAMPLES OF CHEMICAL REACTIONS IN DAILY LIFEBURNING OF CANDLE

A candle is made from a wax which can easily melt and evaporate. Wax is a fairly simple compound consisting of the two elements Carbon and Hydrogen. In lighting a candle, heat is applied to the end of the cotton wick. The heat that comes out of burning it melts the wax. The molten liquid wax rises up the wick by the capillary force. As the liquid wax evaporates a colour less gas is produced. It is this gaseous wax that reacts with the Oxygen in the air resulting in a flame. The flame is made up of very hot molecules.C25H52 + 38 O2 ------- 25CO2 + 26 H2O + energy (heat and light) 

BAKING OF CAKE

Baking is not usually thought of as a chemical industry, but it relies on the interactions of the various chemicals in flour and the other substances used and thus is chemically based.

Usually the properties of the various ingredients are known to the home cook, but not why they behave in that way.2 NaHCO3 → Na2CO3 + H2O + CO2

EXPLOSION OF FIREWORKS

Several chemical reactions occur to make fireworks explode. These include oxidation, reduction, and incandescence. It other words, they burn, explode, and make pretty colours. Oxidation and reduction are really just one reaction: the redox reaction. This is what you see and hear during a fireworks display.KClO4(solid) → KCl(solid) + 2 O2(gas)

COMBUSTION OF WOOD

As wood burns, it turns into a pile of ashes and gases that rise into air. After the wood is burned, it cannot be restored to its original form as a log. The two reaction products are Carbon dioxide CO2 and Water H2O. Both are released as gases in air. Carbon dioxide is a normal component of air; water is generally known as a liquid, but it is a gas at the high temperature of flame. Indeed, gaseous water is a normal component of air (atmospheric humidity). So in wood combustion we observe solid wood disappear and be converted to gas products, leaving only some ashes. These are formed by minor components of wood

that cannot burn and remain solid.

C6

H12

O6

6 O2

6 CO2

6 H2

O

PHOTOSYNTHESIS

The most important chemical reaction helps living things like us live and breathe. That chemical reaction is calledphotosynthesis. This first starts when the sun rays, the light from the sun that makes it to earth goes through this process. When this energy hits the plants the reaction starts and the plant absorbs specific parts of the light. The part of, this chloroplast this is a plastid or a thin covering of cells is a green colored matter of plants, this has chlorophyll molecules and the molecules go into an area called the Stroma. This is a matrix of cells. Chlorophyll it is a green colored matter of plants. This reaction is divided into two groups, the light dependent reaction and light independent reaction. 6CO2 + 6H2O (+ light energy) C6H12O6 + 6O2

CHLORINE BLEACH

Actually the chlorine in the bleach exists as hypochlorous acid. This compound is so oxidative that it has a property of bleaching and sterilization. Gaseous chlorine is generated when hypochlorous acid is mixed with hydrochloric acid. Hydrochloric acid is used in toilet cleaner.HOCl + HCl ↔ H2O + Cl2 ↑

RESPIRATION

Respiration is the reverse of photosynthesis. The body cells in your body force gl ucose and oxygen to mix their atoms up turning into carbon dioxide, also known as co2, and water (h2o). It releases stored energy you can use to run, work, play, learn, and most importantly, live. Respiration is also an important chemical reaction. In fact, it is just as important as

photosynthesis. People and animals cannot live without respiration. C6H12O6+O2=>CO2+H2O+energy

ACID RAIN

Acid rain is a popular term for the atmospheric deposition of acidified rain, snow, sleet, hail, acidifying gases and particles, as well as acidified fog and cloud water. The increased acidity of these depositions, primarily from the strong acids, sulfuric and nitric, is generated as a by-product of the combustion of fuels containing sulfur or nitrogen, especially fossil fuel power plants. Acid rain is formed when SO3 dissolves in water, i.e. in droplets that form clouds. As a product H2SO4 is formed. Since SO3 is produced as a common nusproduct in areas with heavy industry, thus these areas are under particular danger of acid rain. So, instead of pure rainwater, diluted H2SO4 falls on the ground.SO3(g) + H2O(l) ---> H2SO4(aq)

LIGHTING A MATCH

When you light a match, a reaction occurred in the tip of the match that contains potassium chlorate (an oxidant), sulfur, starch, glue and some other ingredients. The striking surface on the match box contains powdered glass, red phosphorus, and glue. When a match strikes on this surface, the heat of the friction causes a reaction between the potassium chlorate in the

match head and the red phosphorus in the striking surface. That way sparks are formed, which finally ignite the match head that continues to burn after that.16KCl3 + 3P4 S3 --> 16 KCl + 9SO2

KIDNEY STONES

Stones are made up of several different substances. Most stones are made of calcium oxalate, calcium urate or calcium pyrophosphate. It has been shown that when you don't consume enough calcium, there is a high chance that the calcium that you do get will bond with oxalate in your gastrointestinal tract. The bonding of these two liquids forms the solid calcium oxalate. This is what the majority of kidney stones are made of.Ca+C204--> CAC2O4

TYPES OF CHEMICAL REACTIONS

Combination reaction or synthesis reaction: it is a reaction in which 2 or more chemical elements or compounds unite to form a more complex product.

Example: N2 + 3 H2 → 2 NH3

Isomerisation reaction: is a reaction in which a chemical compound undergoes a structural rearrangement without any change in its net atomic composition. Example: trans-2-butene and cis-2-butene are isomers.

Chemical decomposition reaction or analysis: is a reaction in which a compound is decomposed into smaller compounds or elements:

Example: 2 H2O → 2 H2 + O2

Single displacement or substitution: this type of reaction is characterized by an element being displaced out of a compound by a more reactive element.

Example: 2 Na(s) + 2 HC1(aq) → 2 NaC1(aq) + H2(g)

Metathesis or Double displacement reaction: represents a reaction in which two compounds exchange ions or bonds to form different compounds

Examples: NaC1(aq) + AgNO3(aq) → NaNO3(aq) + AgC1(s)

NEUTRALISATION REACTION

ACIDAn acid is a species that is a proton donor and the word acid comes from the Latin word 'acidus' meaning sour. When an acid is added to water, it will give a solution with a pH of less than 7, pH is a measure of concentration of hydrogen ions in a solution. Most chemistry laboratories will have common strong acids, such as sulphuric acid (H2SO4), hydrochloric acid (HCl) and nitric acid (HNO3). Strong acids are acids that fully dissociates, and this means that it will be fully ionised in solution. Weak acids, that you are likely to come across every day, includes ethanoic (acetic) acid which is in vinegar, methanoic (formic) acid which is in insect bites, and citric acid which is in citrus fruits). Weak acids are acids that partially dissociates and this means that it will only be partially ionised in solution. All acids contain hydrogen and when an acid is added to water, the acid release H+ ions, also known as protons, into solution and as this happens with all acids, acids are known as proton donors. The H+ ion in an acid is the active ingredient and it is responsible for all reactions. 

BASEBases are species that is a proton acceptor and reacts with acids to form salt and water. A salt is any chemical compound formed when a H+ from an acid has been replaced by a metal ion or any other positive ion. Positively charged ions, such as NH4+ are known as cations and anions are negatively charged ions which derives from an acid in a salt. For example , hydrochloric acid goes to chloride salts, such as sodium chloride.  

REACTIONNeutralization is a chemical reaction between an acid and a base that produces water (H2O) and a salt (Figure 5.16). The salts formed may be soluble in water or can be insoluble. If a salt is insoluble, a precipitate will form. A precipitate is a suspension of small, solid particles formed during a chemical reaction.A neutralization reaction can be summarized as follows:acid + base → salt + waterFor example, the chemical equation for the neutralization reactionHCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

When hydrochloric acid and sodium hydroxide are mixed in solution, theyundergo a neutralization reaction. The products are sodium chloride (table salt) and water.After the actual chemical reaction, the sodium chloride would be dissolved in the water.

Examples of Neutralization reactions

Hydrochloric acid + Sodium hydroxide → Sodium chloride + WaterHCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Sulfuric acid + Magnesium hydroxide → Magnesium sulfate + WaterH2SO4(aq) + Mg(OH)2(aq) → MgSO4(aq) + 2H2O(l)

Nitric acid + Potassium hydroxide → Potassium nitrate + WaterHNO3(aq) + KOH(aq) → KNO3(aq) + H2O(l)

Acetic acid + Sodium hydroxide → Sodium acetate + WaterCH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l)

Hydrochloric acid + Potassium hydroxide → Potassium chloride + WaterHCl(aq) + KOH(aq) → KCl(aq) + H2O(l)

APPLICATIONS OF NEUTRALISATIONNeutralization reactions have commercial uses, such as in pharmaceutical manufacturing, waste treatment, and agriculture. In making pharmaceuticals, the pH of a solution may be changed from acidic to basic (or vice versa) so that the desired product forms a precipitate. The precipitate can then be collected by filtering the products of the reaction. In agriculture, calcium carbonate may be added to acidic soil. The calcium carbonate enters into a neutralization reaction with some of the acid in the soil water, and the soil Ph becomes more basic.

Calcium carbonate,commonly called lime, is added to soilto raise the pH.

Neutralization reactions are also used in medicine. For example, acid reflux is a condition in which stomach acid (HCl) causes discomfort. The symptoms of acid reflux can be treated with antacids, which are composed of bases. They can produce a neutralization reaction with some of the hydrochloric acid in the stomach. Bee stings can also be treated using a neutralization reaction (Figure 5.17). When it stings, a bee releases methanoic acid, which attacks nerves in the skin. Bee stings can be treated with a cream that contains ammonia, which is a base. The ammonia enters into a neutralization reaction with the methanoic acid, which prevents further irritation of the nerve endings.

The methanoic acid in abee sting, also known as formic acid(HCOOH), can be neutralized with anammonia-based cream.

An antacid antiflatuent syrup

The food industry uses neutralization reactions to adjust the pH of products. Packaged and processed foods often have an acidic pH, since harmful bacteria are less likely to grow under these conditions. For example, vinegar (dilute acetic acid) or citric acid may be added to foods to lower the pH. You will find vinegar in the ingredient list of many prepared foods, such as ketchup.

Vinegar is added to packaged foods,such as ketchup, for taste and to prevent harmfulbacteria growing and causing them to spoil.

You may be using acids and bases without realizing it. For example, we often serve fish with a wedge of lemon (Figure 5.19). Fish is a weak base. Lemon juice (an acid) lowers the pH, which eliminates the fishy odour. Lemon juice is also often added to spinach salads. Spinach contains iron, which is an important nutrient for your body. Lemon juice contains vitamin C (ascorbic acid). Nutritionists believe that adding lemon juice will help your body to absorb iron.

A neutralization reaction occurs whenlemon juice is added to fish.

CORROSION

Corrosion of Metals can be defined as the destructive attack of a metal through interaction with its environment.The corrosion of a metal is a chemical process by which the metal is oxidized. The tendency to suffer corrosion attack in a given environment varies with the particular metal.Corrosion is the deterioration of a material due to interaction with its environment. It is the process in which metallic atoms leave the metal or form compounds in the presence of water and gases. Metal atoms are removed from a structural element until it fails, or oxides build up inside a pipe until it is plugged.

CONSEQUENCES OF CORROSIONThe consequences of corrosion are many and varied and the effects of these on the safe, reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal. Failures of various kinds and the need for expensive replacements may occur even though the amount of metal destroyed is quite small. Some of the major harmful effects of corrosion can be summarised as follows: 1. Reduction of metal thickness leading to loss of mechanical strength and structural failure or breakdown. When the metal is lost in localised zones so as to give a crack like structure, very considerable weakening may result from quite a small amount of metal loss. 2. Hazards or injuries to people arising from structural failure or breakdown (e.g. bridges, cars, aircraft). 3. Loss of time in availability of profile-making industrial equipment. 4. Reduced value of goods due to deterioration of appearance. 5. Contamination of fluids in vessels and pipes (e.g. beer goes cloudy when small quantities of heavy metals are released by corrosion). 6. Perforation of vessels and pipes allowing escape of their contents and possible harm to the surroundings. For example a leaky domestic radiator can cause expensive damage to carpets and decorations, while corrosive sea water may enter the boilers of a power station if the condenser tubes perforate. 7. Loss of technically important surface properties of a metallic component. These could include frictional and bearing properties, ease of fluid flow over a pipe surface, electrical conductivity of contacts, surface reflectivity or heat transfer across a surface.

8. Mechanical damage to valves, pumps, etc, or blockage of pipes by solid corrosion products. 9. Added complexity and expense of equipment which needs to be designed to withstand a certain amount of corrosion, and to allow corroded components to be conveniently replaced.

TYPES OF CORROSION

UNIFORM CORROSION

Corrosion of a ferric stainless steel door in a sulphur-containing atmosphere.Uniform or general corrosion is the most classical form of corrosion, but is not always the most important in terms of cost or safety. It is characterized by the existence of several individual electrochemical processes that occur uniformly over the whole of the surface considered.The consequences of uniform corrosion are a decrease in metal thickness per unit time (or a weight loss per unit area per unit time) if the corrosion products are soluble, or a more or less uniform deposit of these products if they are insoluble.

GALVANIC CORROSION

Galvanic corrosion between a copper tube and copper-phosphorus braze in the brazing regionGalvanic corrosion occurs in the presence of an electrolyte such as seawater when dissimilar types of metals join together. Most metals have different electrical potentials. When connected electrically and placed in an electrolyte, the more active metal becomes the anode because it has more negative potential and corrodes faster than if it were alone in the environment. The more noble (less active) metal becomes the cathode because it has more positive potential and corrodes at a slower rate than if it were alone in the

environment. Electrical current flows between the metals until their potentials are equal. Galvanic corrosion typically appears in joints where the two dissimilar metals meet.

CREVICE CORROSION

Crevice corrosion (beneath a seal) on a stainless steel flange exposed to a chloride-rich medium.

Crevice corrosion, also called concentration cell corrosion, forms when a liquid corrosive is trapped in narrow gaps of space between metals, or between non-metals and metals. Aggressive ions like chlorides must be present in the electrolyte. Once the liquid has settled in the gap, a corrosion reaction begins to take place. The reaction consumes the oxygen in the bottom of the gap, and an anodic area develops adjacent to the oxygen-depleted zone. The material on the exterior acts as the cathode. Crevice corrosion develops quite similar to pitting corrosion after the initiation stage with a gradual decrease of the pH and an increase of the chloride concentration within the crevice.

PITTING CORROSION

Tubercle on the surface of a copper tube (corrosion by type I pits in sanitary cold water).

Pitting corrosion causes damage by randomly attacking a limited section of the metal's surface, leaving behind holes that are larger in depth than width. The "pit" that forms, functions as the anode and the metal that is left undamaged functions as the cathode. The corrosion process starts with a chemical breakdown in a small spot such as a scratch or a nick, usually occurring under a surface coating that has experienced wear or damage. As the pitting progresses, the metal becomes thinner, and fatigue and stress corrosion cracking begins at the base of the corrosion pits. The failure can be catastrophic. For example, a gas

line that was positioned over a sewer line killed 215 people in Guadalajara, Mexico, because of a single pit that had formed in the gas line.

INTERGRANULAR CORROSION

Exfoliation corrosion morphology inside a water pipeline.

The microstructure of metals and alloys consists of a granular composition. Grains are small crystals whose surfaces join the surfaces of other grains to form grain boundaries. Grain boundaries separate the grains. Intergranular corrosion, also called intercrystalline corrosion, occurs on or adjacent to the grain boundaries of a metal. Some causes of intergranular corrosion are welding, stress annealing, improper heat treating or overheating in service. Inspectors have difficulty in detecting the early stages of intergranular corrosion. This form of corrosion results in a loss of strength in metal parts where the grains have fallen out.

SELECTIVE CORROSION

Micrographic appearance of a dezincification of brass.This process, also called "dealloying" or "selective leaching", involves the selective dissolution of one of the elements in a single phase alloy or one of the phases in a multiphase alloy. The most well known example is the dezincification of brass (e.g. 70Cu - 30Zn). In this case, the brass takes on a red coppery tinge as the zinc is removed. It also becomes porous and very brittle, without modification to the overall dimensions of the part.

EROSION CORROSION

"Horseshoe" type erosion-corrosion damage in a copper pipeline.Erosion-corrosion, also known as flow-assisted corrosion, is the process of moving a corrosive liquid against a metal surface, leading to the accelerated loss of material. For example, erosion-corrosion can occur when a ship's propeller churns in the ocean. The damage to a metal surface appears as waves, rounded holes, and/or horseshoe-shaped grooves in a pattern on the surface, indicating the direction of flow of the corrosive liquid. Soft alloys are more susceptible to erosion-corrosion.

CAVITATION CORROSION

Cavitation of a nickel alloy pump impeller blade exposed to a hydrochloric acid medium.Cavitation-corrosion is particular form of erosion caused by the "implosion" of gas bubbles on a metal surface. It is often associated with sudden variations in pressure related to the hydrodynamic parameters of the fluid (e.g. hydraulic turbine blades, propellers, stirrer blades, etc.). A regular hydraulic regime in the fluid is extremely important. A good surface condition decreases the number of potential sites for the formation of vapour bubbles. An increase in fluid pressure is often sufficient to maintain a single phase fluid, thus avoiding the formation of vapour bubbles. Plastic or rubber coatings have often proved to be effective, although the problems of adherence between the coating and the metal are frequently an obstacle. 

FRETTING CORROSION

Friction-wear at an axle-cylinder contact point.Fretting-corrosion is a combined damage mechanism involving corrosion at points where two moving metal surfaces make rubbing contact. It occurs essentially when the interface is subjected to vibrations (repeated relative movement of the two contacting surfaces) and to compressive loads. The amplitude of the relative movement is very small, typically of the order of a few microns. When the frictional movement in a corrosive medium is continuous, the resulting process is termed tribocorrosion.

STRESS CORROSION CRACKING

Stress corrosion cracking of an alloy steel welded assembly (welding constraints)Stress corrosion cracking (SCC) is a complex form of corrosion that occurs when brittle, dry cracks develop from the combined effects of a tensile stress (A stress caused by bending or stretching a material causing two sides of the material to pull apart) and a corrosive environment. Stress corrosion affects only a section of material; leaving most of the material unaffected. Causes of stress corrosion cracking include cold deformation and forming, welding, heat treatment, machining, and grinding. The brittle cracks associated with stress corrosion cracking can have intergranular or transgranular morphology and can occur in combination with hydrogen embrittlement. Inspectors often underestimate stress corrosion cracking. Due to the difficulty in discovering the fine cracks, engineers class stress corrosion cracking as a catastrophic form of corrosion. The damage cannot be predicted. In most cases, SCC coincides with the process of active path corrosion (APC), whereby the corrosive attack or anodic dissolution initiates at specific paths within the material. In some cases, these paths lie along grain boundaries. In other cases, the path lies along a specific crystallographic boundary within the grains. Alloy composition strongly affects stress corrosion cracking, as does the concentration of specific corrodent species, and to a lesser degree, stress intensity.

 FATIGUE CORROSION

Corrosion Fatigue of Copper Tubes in a Subcooler

Corrosion fatigue is the process of metal cracking prematurely in the presence of a corrosive environment, such as saltwater. The combined effects of cyclic stress and corrosion cause this type of failure. Corrosion fatigue occurs when metal breaks at stresses significantly lower than its tensile strength. Characterized by little or no branching, this corrosive process is accelerated when the fatigue causes the protective passive film to split. The environment in which the metal resides influences the beginning of the cracks and/or their growth rate. Inspectors have difficulty detecting corrosion fatigue because the cracks can be very fine and unnoticeable; stressed coupons, which are metal samples, have been used for monitoring and testing purposes to detect failure. The end results of corrosion fatigue are catastrophic if not detected. Types of stresses that contribute to fatigue failure include applied stresses, thermal expansion and contraction, poor assembly, welding, casting, soldering, mechanical cleaning, heat treatment, forming, shaping, cutting, and shearing.

FILIFORM CORROSION

Filiform corrosion on coated aluminium

Filiform corrosion is a form of concentration cell corrosion on metallic surfaces coated with a thin organic film. Typically, the surface must be somewhat acidic and the relative humidity in the environment must be 65 to 90 percent. This type of corrosion usually begins in a defect within a surface's protective coating. Filaments of corrosion products begin to grow

and cause the protective coating to degrade. The filaments take on the appearance of thin threads that lie long branching paths that expand out from the original corrosion site. Why the concentration cell breaks its original isotropic symmetry and becomes threadlike and spreads is unclear. Filiform corrosion is unique because it only affects the appearance of the surface; it does not weaken the metallic material.

PREVENTION OF CORROSION

Corrosion of metals can be prevented if the contact between metal and air is cut off. This is done in a number of ways. Some of the methods are given below:

Corrosion can be prevented if the metal is coated with something which does not allow moisture and oxygen to react with it.

Coating of metals with paint, oil, grease or varnish prevents the corrosion of metals.

Coating of corrosive metals with non-corrosive metals also prevents corrosion. Some of the methods by which metals can be coated with non-corrosive metals are:

Galvanizing: It is process of giving a thin coating of zinc on iron or steel to protect them from corrosion. Iron is galvanized by dipping it in molten zinc. It is then taken out and allowed to cool. Galvanizing is an effective methods of protecting steel

because even if the surface is scratched, the zinc still protects the underlying layer.

Tinning: It is the process of giving a coating of tin, i.e., molten tin. Cooking vessels, made of copper and brass get a greenish coating due to corrosion. This greenish coating is poisonous. Therefore they are given a coating of tin to prevent corrosion.

Electroplating: In this method of a metal is covered with another metal using electrolysis. Silver-plated spoons, gold-plated jewellery, etc, are electroplated.

Anodizing: In this method metals like copper and aluminium are electrically coated with a thin strong film of their oxides. This film protects the metals from corrosion.

Alloying: Corrosion can be also prevented by alloying some metals with other metals. The resultant metals called alloys do not corrode easily, e.g. stainless steel.

RANCIDITYRancidity is a very general term and in its most general meaning, it refers to the spoilage of a food in such a way that it becomes undesirable (and usually unsafe) for consumption. When people say that a food has "gone bad," what they're usually talking about is rancidity. Most of the time, but not always, rancidity can change the odors or flavors of a food in such a way that it becomes very unpleasant to smell or taste.

While most any food can technically become rancid, this term applies particularly to oils. Oils can be especially susceptible to rancidity because their chemistry can make them exceptionally susceptible to oxygen damage.

HYDROLYTIC RANCIDITY

In hydrolytic rancidity, the fatty acids of a fat or oil molecule split from the glycerol backbone in the presence of water. This hydrolysis may release volatile free fatty acids that

readily vaporize into the air. Butyric acid is an example of a volatile fatty acid, giving rise to the characteristic offensive odour and taste of rancid butter. Hydrolytic rancidity occurs in dairy products because of the specific types of fatty acids found in milk fats. Although foods that have undergone hydrolytic rancidity may be safe to eat, the danger in these foods stems from the foul quality the rancid fats impart to your food.

OXIDATIVE RANCIDITY

Oxidative rancidity occurs when fatty acids are exposed to oxygen in the presence of heat or light, resulting in the formation of hydro peroxide compounds. These hydro peroxides in turn form aldehyde molecules. Oxygenated aldehydes are toxic compounds that cause oxidative stress in the cells of your body and may increase your risk of degenerative illness and atherosclerotic disease. Hydro peroxide fatty acids may also have a detrimental effect on the fat-soluble vitamins A and E in your foods. Consuming fats and oils that have undergone oxidative rancidity may be dangerous to your health.

MICROBIAL RANCIDITY

Another degradation process is microbial rancidity, in which micro-organisms such as bacteria, moulds and yeast use their enzymes to break down chemical structures in the oil, producing unwanted odours and flavours. Water needs to be present for microbial growth to occur.

PREVENTION OF RANCIDITYAntioxidants are often added to fat-containing foods in order to retard the development of rancidity due to oxidation. Natural anti-oxidants include flavonoids, polyphenols, ascorbic acid (vitamin C) and tocopherols (vitamin E). Synthetic antioxidants include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl 3,4,5-trihydroxybenzoate also known as propyl gallate and ethoxyquin. The natural antioxidants tend to be short-lived, so synthetic antioxidants are used when a longer shelf life is preferred. The effectiveness of water-soluble antioxidants is limited in preventing direct oxidation within fats, but is valuable in intercepting free radicals that travel through the watery parts of foods. A combination of water-soluble and fat-soluble antioxidants is ideal, usually in the ratio of fat to water.In addition, rancidification can be decreased, but not completely eliminated, by storing fats and oils in a cool, dark place with little exposure to oxygen or free radicals, since heat and light accelerate the rate of reaction of fats with oxygen. (Oxidative rancidity or auto-oxidation is a chemical reaction with low activation energy consequently the rate of reaction is not significantly reduced by cold storage).Do not add fresh oil to vessels containing old oil. The old oil will trigger a reaction and the new oil will become rancid far more rapidly than if the oil was stored in a clean empty vessel. Avoid using vessels that are wet, this will also speed up the problems associated with oxidation, allow tanks to drain and dry adequately before use.

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